Method of formation of channels for a sonar, in particular for a towed-array sonar

Abstract

In a method of formation of channels for a sonar, after having sampled at a frequency T=1/4f 0 (where f 0 is the receiving center frequency of the sonar) the signals from the hydrophones of the sonar and having translated them to baseband, the signals thus translated are subsampled with a period T SE =kT (wherein k is an integer) substantially equal to 1.25 B, where B is the reception bandwidth of the sonar. A first set of signals is subsampled at identical times to form a frontal sector. Two further sets of signals are subsampled with delays between the signals from two adjacent hydrophones equal to T, which determines two side sectors adjacent to the frontal sector. The subsampled signals are then transmitted serially by the towing cable of the sonar device towed array and are processed in FFT circuits which allow to form in each sector a set of channels covering the sector. This allows to considerably reduce the data transmission rate between the towed portion of the sonar and the portion located in the towing ship.

What is claimed is: 
     
       1. A method of formation of channels for a sonar device for monitoring a sector of observation, in particular for a towed-array sonar device, said sonar device including an array of hydrophones delivering received signals, wherein said received signals are delayed to divide said sector of observation into a central sector and several separate sectors adjacent to said central sector, then the signals thus delayed are phase-shifted to form in each sector a set of channels, which method comprises: sampling said signals from the hydrophones with a period T which is a function of the center frequency of reception f 0  of the sonar device and equal to the delay between two consecutive hydrophones, said delay corresponding to said two sectors adjacent to the central sector;   translating to baseband the signals thus sampled;   subsampling the signals thus translated with a period T SE  =kT (where k is an integer) which is a function of the reception bandwidth B of the sonar device and selecting a set of N signals with the same rank determining said central sector, and shifting the sets of signals with respect to each other by a delay multiple of T determining said adjacent sectors;   serially transmitting the subsampled signals;   separating the transmitted signals corresponding to each sector; and   forming in each sector a set of channels covering said sector.   
     
     
       2. A method according to claim 1, wherein T= 1/4 f 0 , the translation being performed in accordance with the digital complex demodulation technique, and wherein T SE  is substantially equal to 1.25 B. 
     
     
       3. A method according to claim 2, which comprises forming the channels of each sector by using a device implementing a fast Fourier transformation. 
     
     
       4. A method according to claim 3, which comprises utilizing a plurality of channels which exceeds the limits of the sector of interest so as to implement the transformation, wherein end channels which overlap adjacent channels are eliminated.